System and method for battery charging

ABSTRACT

A charging circuit includes an N-channel metal-oxide-semiconductor field-effect transistor (NMOSFET) that controls a charging current to a battery, a charge pump that generates a driving signal based on a plurality of pulses, and a resistor coupled to a gate of the NMOSFE. The resistor and a capacitance of the gate of the NMOSFET form a low pass filter. The driving signal is filtered by the low pass filter to control a gate voltage of the NMOSFET. A variation of a gate-source voltage of the NMOSFET is proportional to a pulse density of the plurality of pulses. A variation of the charging current flowing through the NMOSFET to the battery is proportional to the pulse density.

RELATED APPLICATION

This application is a continuation of the co-pending U.S. application,Ser. No. 11/827,144, titled “System and Method for Battery Charging,”filed Jul. 5, 2007, which is hereby incorporated by reference.

BACKGROUND

Battery pre-charging can be enabled when battery voltage is low. Apre-charging current is relatively small compared to a normal chargingcurrent during normal charging. Conventional battery charging systemsperform battery pre-charging by controlling a switch coupled in serieswith a current limiting resistor. Such battery charging systems arecostly. In addition, such battery charging systems have large powerdissipation and low efficiency during pre-charging.

SUMMARY

A charging circuit includes an N-channel metal-oxide-semiconductorfield-effect transistor (NMOSFET) that controls a charging current to abattery, a charge pump that generates a driving signal based on aplurality of pulses, and a resistor coupled to a gate of the NMOSFE. Theresistor and a capacitance of the gate of the NMOSFET form a low passfilter. The driving signal is filtered by the low pass filter to controla gate voltage of the NMOSFET. A variation of a gate-source voltage ofthe NMOSFET is proportional to a pulse density of the plurality ofpulses. A variation of the charging current flowing through the NMOSFETto the battery is proportional to the pulse density.

BRIEF DESCRIPTION OF THE DRAWINGS

Features and advantages of embodiments of the claimed subject matterwill become apparent as the following Detailed Description proceeds, andupon reference to the drawings, wherein like numerals depict like parts,and in which:

FIG. 1A shows a block diagram of a battery charging system, inaccordance with one embodiment of the present invention.

FIG. 1B shows a block diagram of a battery charging system, inaccordance with one embodiment of the present invention.

FIG. 2 shows a flowchart of operations performed by a battery chargingsystem, in accordance with one embodiment of the present invention.

FIG. 3 shows a flowchart of operations performed by a battery chargingsystem, in accordance with one embodiment of the present invention.

DETAILED DESCRIPTION

Reference will now be made in detail to the embodiments of the presentinvention. While the invention will be described in conjunction withthese embodiments, it will be understood that they are not intended tolimit the invention to these embodiments. On the contrary, the inventionis intended to cover alternatives, modifications and equivalents, whichmay be included within the spirit and scope of the invention as definedby the appended claims.

Furthermore, in the following detailed description of the presentinvention, numerous specific details are set forth in order to provide athorough understanding of the present invention. However, it will berecognized by one of ordinary skill in the art that the presentinvention may be practiced without these specific details. In otherinstances, well known methods, procedures, components, and circuits havenot been described in detail as not to unnecessarily obscure aspects ofthe present invention.

FIG. 1A shows a block diagram of a battery charging system 100, inaccordance with one embodiment of the present invention. As shown inFIG. 1 A, a charger 110 can be used to charge a battery pack 102 havinga plurality of cells 102_1-102 _(—) n. In one embodiment, a chargingcircuit 170 coupled to the battery pack 102 and the charger 110 controlsa charging current flowing from the charger 110 to the battery pack 102by controlling a charging switch 130. A discharging switch 132 can beused to control a discharging current during discharging. Duringcharging, the discharging switch 132 can be on or off. If thedischarging switch 132 is off, a charging current can flow from thecharger 110 to the battery pack 102 through a body diode 134 of thedischarging switch 132.

In one embodiment, the charging circuit 170 includes a pulse generator140 for generating a plurality of pulses 142 to control the chargingswitch 130, and a controller 120 coupled to the pulse generator 140 forcontrolling a pulse density of the plurality of pulses 142. If N pulsesare generated by the pulse generator 140 during a time period T, thepulse density D_(p) is equal to the number of pulses N divided by thetime period T (D_(p)=N/T). Advantageously, a charging current flowingthrough the charging switch 130 can be adjusted according to the pulsedensity of the plurality of pulses 142, in one embodiment. In oneembodiment, the charging switch 130 can include, but is not limited to atransistor, e.g., an n-type metal-oxide-semiconductor field-effecttransistor.

In one embodiment, the pulse generator 140 is a PDM (pulse densitymodulation) pulse generator. The PDM pulse generator 140 can be used togenerate a plurality of normally distributed pulses 142 over a timeperiod. The PDM pulse generator 140 can have numerous configurations.Advantageously, the controller 120 controls a pulse density of theplurality of PDM pulses 142 generated by the PDM pulse generator 140, inone embodiment.

A charge pump 150 coupled to the PDM pulse generator 140 can be used toreceive the plurality of PDM pulses 142 from the PDM pulse generator 140and to generate a driving signal 152 which controls the charging switch130. More specifically, the charge pump 150 receives an input voltageVcc and the plurality of PDM pulses 142, and generates the drivingsignal 152 that controls a gate voltage of the charging switch 130,thereby controlling a conductance of the charging switch 130. In oneembodiment, the voltage of the driving signal 152 is higher than thevoltage of the input signal Vcc, and the voltage of the driving signal152 is sufficient to conduct the charging switch 130 during charging.The charge pump 150 can be a single stage charge pump with numerousconfigurations or a multi-stage charge pump with numerousconfigurations, in one embodiment.

Advantageously, in one embodiment, the controller 120 monitors a batteryvoltage of the battery pack 102 (and/or cell voltages of cells 102_1-102_(—) n) and a charging current, and controls the pulse density of theplurality of PDM pulses 142 generated by the PDM pulse generator 140.The charge pump 150 receives the plurality of PDM pulses 142 andgenerates a driving signal 152 to control a conductance of the chargingswitch 130, in one embodiment. Therefore, the conductance of thecharging switch 130 can be controlled according to the pulse density ofthe PDM pulses 142. Accordingly, the charging current flowing throughthe charging switch 130 can be adjusted by controlling the pulse densityof the plurality of PDM pulses 142.

As show in FIG. 1A, the charging switch 130 has a gate 130 g, a source130 s, and a drain 130 d. The charging current flowing through thecharging switch 130 is equal to the drain-source current I_(DS) of thecharging switch 130, in one embodiment. According to characteristics ofthe charging switch 130 (e.g., an n-type metal-oxide-semiconductorfield-effect transistor) during an active (linear) region, an incrementof the drain-source current ΔI_(DS) during a time period T is given by:

ΔI _(DS)=2K( V _(GS) −V _(T))ΔV _(GS) +K(ΔV _(GS))²≈2K( V _(GS) −V_(T))ΔV _(GS),   (1)

where V_(T) represents a threshold voltage of the charging switch 130,ΔV_(GS) represents an increment of the gate-source voltage of thecharging switch 130 during the time period T, V_(GS) represents anaverage gate-source voltage of the charging switch 130 during the timeperiod T, and K represents a parameter of the charging switch 130, whichis related to the fabrication process. As shown in equation (1), theincrement of the drain-source current ΔI_(DS) during the time period Tis proportional to the increment of the gate-source voltage ΔV_(GS)during the time period T. Consequently, the charging current can beadjusted by controlling the gate-source voltage of the charging switch130. In one embodiment, a resistor 126 coupled to the charging switch130 is used as a low pass filter to reduce fluctuation of thegate-source voltage of the charging switch 130.

According to characteristics of the charge pump 150, a voltage increaseΔV₁ at the gate 130 g of the charging switch 130 during the time periodT can be given by:

$\begin{matrix}{{{\Delta \; V_{1}} = \frac{\eta \; {NC}_{150}V_{CC}}{C_{130g}}},} & (2)\end{matrix}$

where N represents the number of PDM pulses 142 received by the chargepump 142 during the time period T, C₁₅₀ represents a capacitance of acharge pump flying capacitor in the charge pump 150, Vcc represents aninput voltage of the charge pump 150, C_(130g) represents a capacitanceof a gate capacitor at gate 130 g of the charging switch 130, and ηrepresents an efficiency of the charge pump 150 (in one embodiment, fora single stage charge pump, η=1, for a multi-stage charge pump, 0<η<1).

In one embodiment, a pull-down resistor 124 is coupled to the gate 130 gand the source 130 s of the charging switch 130. As such, during thetime period T, a current will flow from the gate capacitor (at gate 130g) to source 130 s via the pull-down resistor 124. Therefore, there is avoltage decrease ΔV₂ at the gate 130 g of the charging switch 130 duringtime period T, which can be given by:

$\begin{matrix}{{{\Delta \; V_{2}} = {\frac{Q_{130g}}{C_{130g}} = \frac{\frac{\overset{\_}{V_{GS}}}{R_{124}}T}{C_{130g}}}},} & (3)\end{matrix}$

where Q_(130g) represents a total electric quantity decreased at thegate capacitor during the time period T, and R₁₂₄ represents aresistance of the pull-down resistor 124.

Accordingly, the total increment of the gate voltage (at gate 130 g) ofthe charging switch 130 during the time period T is equal to ΔV₁−ΔV₂. Inone embodiment, the total increment of the gate-source voltage ΔV_(GS)of the charging switch 130 during the time period T is equal to thetotal increment of the gate voltage (at gate 130 g) of the chargingswitch 130. Accordingly, the total increment of the gate-source voltageΔV_(GS) of the charging switch 130 during the time period T can be givenby:

$\begin{matrix}{{\Delta \; V_{GS}} = {{{\Delta \; V_{1}} - {\Delta \; V_{2}}} = {\frac{{{NC}_{150}V_{CC}} - {\frac{\overset{\_}{V_{GS}}}{R_{124}}T}}{C_{130g}}.}}} & (4)\end{matrix}$

Since the pulse density D_(P) of the PDM pulses 142 is equal to thenumber of the PDM pulses N divided by the time period T, equation (4)becomes:

$\begin{matrix}\begin{matrix}{{\Delta \; V_{GS}} = {{\Delta \; V_{1}} - {\Delta \; V_{2}}}} \\{= \frac{{{NC}_{150}V_{CC}} - {\frac{\overset{\_}{V_{GS}}}{R_{124}}T}}{C_{130g}}} \\{= {\frac{{D_{P}{TC}_{150}V_{CC}} - {\frac{\overset{\_}{V_{GS}}}{R_{124}}T}}{C_{130g}}.}}\end{matrix} & (5)\end{matrix}$

Therefore, the total increment of the gate-source voltage ΔV_(GS) of thecharging switch 130 during the time period T is proportional to thepulse density D_(P) of the PDM pulses 142 during the time period T. Asdescribed above, the increment of the charging current during the timeperiod T is proportional to the increment of the gate-source voltageΔV_(GS) during the time period T. As a result, the increment of thecharging current of the charging switch 130 during the time period T isproportional to the pulse density D_(P) of the PDM pulses 142 during thetime period T.

Advantageously, the charging current can be adjusted by controlling thepulse density of the PDM pulses 142. More specifically, the chargingcurrent increases when the pulse density of the PDM pulses 142 increasesand the charging current decreases when the pulse density of the PDMpulses 142 decreases.

In one embodiment, the controller 120 can also enable an oscillator 180which generates a plurality of clock pulses (clock signal) 144 having aconstant frequency. As such, a pulse density of the clock pulses 144 isconstant. The charge pump 150 receives the clock pulses 144 andgenerates a driving signal 152 which can fully turn on the chargingswitch 130, in one embodiment.

The oscillator 180 enabled by the controller 120 can also generate aplurality of clock pulses 146 to control a charge pump 160. In oneembodiment, the charge pump 160 is used to receive the plurality ofclock pulses 146 and generate a driving signal 162 to control thedischarge switch 132.

FIG. 1B shows a block diagram of a battery charging system 100′, inaccordance with one embodiment of the present invention. Elements thatare labeled the same as in FIG. 1A have similar functions and will notbe repetitively described herein for purposes of brevity and clarity.

As shown in the example of FIG. 1B, the controller 120 includes an A/D(analog-to-digital) converter 172, an A/D converter 174, and a processor178. In one embodiment, the A/D converter 172 monitors all the cellvoltages for cells 102_1-102 _(—) n during each cycle (time period T).More specifically, the A/D converter 172 receives a voltage monitoringsignal indicative of a cell voltage for each cell of the plurality ofcells 102_1-102 _(—) n via a multiplexer 190 during each cycle. In oneembodiment, the A/D converter 174 monitors a charging current duringeach cycle (time period T). More specifically, the A/D converter 174receives a current monitoring signal indicative of a battery chargingcurrent via a sense resistor 180. In one embodiment, a processor 178(e.g., a micro-processor) receives monitoring signals from the A/Dconverter 172 and the A/D converter 174, and adjusts the pulse densityof PDM pulses during each cycle (time period T).

Advantageously, the controller 120 monitors a charging current of thebattery pack 102 and a cell voltage for each cell of the plurality ofcells 102_1-102 _(—) n, and controls the charging current of the batterypack 102. In one embodiment, the processor 178 enables the PDM pulsegenerator 140 and controls the pulse density of the PDM pulses duringpre-charging (e.g., pre-charging can be performed when a battery voltagefor the battery pack 102 is less than a predetermined voltage thresholdV_(pre′) or when a cell voltage is less than a predetermined voltagethreshold V_(pre)) such that a pre-charging current I_(pre)(I2<I_(pre)<I1) flows to the battery pack 102. The charging switch 130is controlled linearly (that is, the charging switch 130 is operated inthe active region) by the driving signal 152. Advantageously, theprocessor 178 decreases the pulse density when the charging current isgreater than a first predetermined threshold I1. The processor 178increases the pulse density when the charging current is less than asecond predetermined threshold I2 that is less than the firstpredetermined threshold I1. As such, the battery charging system 100′ isable to pre-charge the battery pack 102 when the battery voltage is lowor zero, in one embodiment.

In one embodiment, the processor 178 disables the PDM pulse generator140 and enables the oscillator 180 during normal charging (e.g., normalcharging can be performed when all the cell voltages of the plurality ofcells 102_1-102 _(—) n are greater than the predetermined thresholdV_(pre)) such that a normal charging current I_(nor) flows to thebattery pack 102. In one embodiment, the normal charging current I_(nor)is greater than the pre-charging current I_(pre). The charging switch130 is fully turned on during normal charging, in one embodiment.

Alternatively, the processor 178 can also enable the PDM pulse generator140 instead of oscillator 180 and controls the pulse density of the PDMpulses during normal charging, in one embodiment. More specifically, theprocessor 178 increases the pulse density of the PDM pulses 142, suchthat the voltage of the driving signal 152 is sufficient to fully turnon the charging switch 130. Therefore, a normal charging current I_(nor)can be provided to the battery pack 102.

In one embodiment, the controller 120 also performs battery protectionwhich includes, but is not limited to, over-voltage protection,over-current protection, under-voltage protection, over-temperatureprotection.

FIG. 2 shows a flowchart 200 of operations performed by a batterycharging system, in accordance with one embodiment of the presentinvention. FIG. 2 is described in combination with FIG. 1A and FIG. 1B.

In block 202, the charger 110 is plugged in, such that the battery pack102 is coupled to the charger 110. In block 204, the battery chargingsystem monitors a cell voltage V_(cell) for each cell 102_1-102 _(—) n.More specifically, the A/D converter 172 converts a voltage monitoringsignal indicative of the cell voltage V_(cell) for each cell 102_1-102_(—) n to a digital signal, and sends the converted digital signal tothe processor 178.

In block 206, the cell voltage V_(cell) for each cell 102_1-102 _(—) nis compared with a predetermined voltage threshold V_(pre). If any cellhas a cell voltage V_(cell) which is less than the predetermined voltagethreshold V_(pre), the flowchart 200 goes to block 210 to performpre-charging. Otherwise, the flowchart 200 goes to block 206 to performnormal charging. The detailed operation of the normal charging isomitted herein for purposes of brevity and clarity.

During pre-charging, the PDM pulse generator 140 is enabled by thecontroller 120 and the oscillator 180 is disabled. The battery 102 canbe charged by a constant power (that is, the charging power for thebattery pack 102 is constant) or it can be charged by a constant current(that is, the charging current for the battery pack is constant), eitherof which can be selected by a user before charging, in one embodiment.In block 210, if constant power charging is selected, the flowchart 200goes to block 212 to perform a constant power charging. In block 212, apredetermined pre-charging current flowing to the battery pack 102 canbe given by:

$\begin{matrix}{{I_{pre} = \frac{P_{pre}}{V_{pre} - V_{cell\_ min}}},} & (6)\end{matrix}$

where P_(pre) represents a constant preset power level for charging thebattery pack 102, which can be defined and programmed by the user beforecharging, and V_(cell) _(—) _(min) represents the lowest cell voltageamong all the cell voltages for cells 102_1-102 _(—) n.

In block 210, if constant power charging is not selected, the flowchart200 goes to block 214 to perform constant current charging. In block214, a predetermined pre-charging current flowing to the battery pack102 can be given by:

I_(pre)=I_(pre) _(—) _(set),   (7)

where I_(pre) _(—) _(set) is a preset constant current level which canbe defined and programmed by the user before charging.

In block 216, a charging current I_(sen) monitored from the senseresistor 180 is compared with a first predetermined threshold I1 and asecond predetermined threshold I2. More specifically, the processor 178receives a current monitoring signal indicative of the charging currentfrom the sense resistor 180, and compares the current monitoring signalwith the first predetermined threshold I1 and the second predeterminedthreshold I2. In one embodiment, the first predetermined threshold I1and the second predetermined threshold I2 are given by:

I1=I _(pre) +I _(hys) ,I2=I _(pre) −I _(hys),   (8)

where I_(hys) represents a hysteresis value which can be used to reduceoscillation of the charging current.

In block 216, if the monitored charging current I_(sen) is greater thanthe second predetermined threshold I2 and less than the firstpredetermined threshold I1 (I2<I_(sen)<I1), the flowchart 200 goes toblock 222. In block 222, the pulse density is unchanged. Morespecifically, the processor 178 maintains the pulse density of the PDMpulses generated by the PDM pulse generator 140 in order to keep thecharging current.

Otherwise, if the monitored charging current I_(sen) is greater than thefirst predetermined threshold I1 (I_(sen)>I1), the flowchart 200 goes toblock 220. In block 220, the pulse density is decreased. Morespecifically, the processor 178 decreases the pulse density of the PDMpulses 142 generated by the PDM pulse generator 140 in order to reducethe charging current.

Otherwise, if the monitored charging current I_(sen) is less than thesecond predetermined threshold I2 (I_(sen)<I2), the flowchart 200 goesto block 218. In block 218, the pulse density is increased. Morespecifically, the processor 178 increases the pulse density of the PDMpulses 142 generated by the PDM pulse generator 140 in order to increasethe charging current.

Advantageously, by adjusting the pulse density of the PDM pulses 142,the charging current can be controlled within a predetermined range.More specifically, the charging current can be controlled such that thecharging current is less than a first predetermined threshold I1(I1=I_(pre)+I_(hys)) and is greater than a second predeterminedthreshold I2 (I2=I_(pre)−I_(hys)). The hysteresis value I_(hys) is usedto reduce oscillation of the charging current, in one embodiment.

In block 224, if a cycle (time period T) is finished, the flowchart 200returns to block 204. Any repetitive description following block 204that has been described above will be omitted herein for purposes ofclarity and brevity. Otherwise, the flowchart 200 returns to block 224.Accordingly, the processor 178 adjusts the pulse density during eachcycle (time period T).

FIG. 3 shows a flowchart 300 of operations performed by a batterycharging system, in accordance with one embodiment of the presentinvention. FIG. 3 is described in combination with FIG. 1A and FIG. 1B.

In block 302, the battery charging system generates a plurality ofpulses 142 by a pulse generator 140 (e.g., a PDM pulse generator). Inblock 304, the battery charging system monitors a charging currentflowing to the battery pack 102. The battery charging system can alsomonitor a battery voltage and/or individual cell voltages for theplurality of cells 102_1-102 _(—) n in the battery pack 102.

In block 306, the battery charging system controls a pulse density ofthe plurality of pulses. More specifically, the battery charging systemdecreases the pulse density when the charging current is greater than afirst predetermined threshold I1. The battery charging system increasesthe pulse density when the charging current is less than a secondpredetermined threshold I2 that is less than the first predeterminedthreshold I1.

In block 308, the battery charging system controls a conductance of acharging switch 130 according to the pulse density. Accordingly, thecharging current flowing through the charging switch 130 to the batterypack 102 can be adjusted according to the pulse density of the pulses142 as shown in block 310. Advantageously, the battery charging currentcan be controlled such that the battery charging current is less thanthe first predetermined threshold I1 and greater than the secondpredetermined threshold I2.

Accordingly, a battery charging system is provided. In one embodiment,the battery charging system adjusts a charging current by controlling apulse density of a plurality of pulses. Advantageously, in oneembodiment, an n-channel metal oxide field effect transistor can be usedas a charging switch, which saves costs and reduces power dissipation.Furthermore, the battery charging system is able to charge the batterywhen a battery voltage is low or zero, in one embodiment.

While the foregoing description and drawings represent embodiments ofthe present invention, it will be understood that various additions,modifications and substitutions may be made therein without departingfrom the spirit and scope of the principles of the present invention asdefined in the accompanying claims. One skilled in the art willappreciate that the invention may be used with many modifications ofform, structure, arrangement, proportions, materials, elements, andcomponents and otherwise, used in the practice of the invention, whichare particularly adapted to specific environments and operativerequirements without departing from the principles of the presentinvention. The presently disclosed embodiments are therefore to beconsidered in all respects as illustrative and not restrictive, thescope of the invention being indicated by the appended claims and theirlegal equivalents, and not limited to the foregoing description.

1. A charging circuit for charging a battery, said charging circuitcomprising: an N-channel metal-oxide-semiconductor field-effecttransistor (NMOSFET) that controls a charging current to a battery; acharge pump that generates a driving signal based on a plurality ofpulses; and a resistor coupled to a gate of said NMOSFET, wherein saidresistor and a capacitance of said gate of said NMOSFET form a low passfilter, wherein said driving signal is filtered by said low pass filterto control a gate voltage of said NMOSFET, wherein a variation of agate-source voltage of said NMOSFET is proportional to a pulse densityof said plurality of pulses, and wherein a variation of said chargingcurrent flowing through said NMOSFET to said battery is proportional tosaid pulse density.
 2. The charging circuit of claim 1, furthercomprising: a pulse generator that generates said plurality of pulses;and a controller, coupled to said pulse generator, that controls saidpulse density of said plurality of pulses according to a status of saidbattery.
 3. The charging circuit as claimed in claim 2, wherein saidpulse density decreases when said charging current is greater than afirst predetermined threshold.
 4. The charging circuit as claimed inclaim 3, wherein said pulse density increases when said charging currentis less than a second predetermined threshold that is less than saidfirst predetermined threshold.
 5. The charging circuit as claimed inclaim 2, wherein said controller controls said pulse density when abattery voltage of said battery charged by said charging current is lessthan a predetermined voltage threshold.
 6. The charging circuit asclaimed in claim 2, wherein said controller controls said pulse densitywhen a cell voltage for each cell of a plurality of cells charged bysaid charging current is less than a predetermined voltage threshold. 7.The charging circuit as claimed in claim 2, further comprising: anoscillator that generates a plurality of clock pulses, wherein saidcharge pump receives said plurality of clock pulses and generates saiddriving signal which turns on said NMOSFET.
 8. The charging circuit ofclaim 7, wherein said charging circuit powers off said oscillator andpowers on said pulse generator when a voltage of said battery is lessthan a threshold, and wherein said charging circuit powers off saidpulse generator and powers on said oscillator when said voltage isgreater than said threshold.
 9. An electronic device comprising: acharger that charges a battery; and a charging circuit, coupled to saidbattery and said charger, that controls a charging current from saidcharger to said battery, said charging circuit comprising: a pulsegenerator that generates a plurality of pulses to control an N-channelmetal-oxide-semiconductor field-effect transistor (NMOSFET) coupledbetween said charger and said battery; a charge pump, coupled to saidpulse generator, that receives said plurality of pulses and generates adriving signal according to said pulses; and a low pass filter coupledto said charge pump, wherein said driving signal is filtered by said lowpass filter to control a gate voltage of said NMOSFET, wherein avariation of a gate-source voltage of said NMOSFET is proportional to apulse density of said plurality of pulses, and wherein a variation ofsaid charging current flowing through said NMOSFET to said battery isproportional to said pulse density.
 10. The electronic device of claim9, further comprising: a controller, coupled to said pulse generator,that controls said pulse density of said plurality of pulses accordingto a status of said battery.
 11. The electronic device of claim 9,wherein said low pass filter is formed by a resistor coupled to a gateof said NMOSFET and a capacitance of said gate of said NMOSFET.
 12. Theelectronic device of claim 9, wherein said pulse density decreases whensaid charging current is greater than a first predetermined threshold.13. The electronic device of claim 12, wherein said pulse densityincreases when said charging current is less than a second predeterminedthreshold that is less than said first predetermined threshold.
 14. Theelectronic device of claim 10, wherein said controller controls saidpulse density when a battery voltage of said battery charged by saidcharging current is less than a predetermined voltage threshold.
 15. Theelectronic device of claim 10, wherein said controller controls saidpulse density when a cell voltage for each cell of a plurality of cellscharged by said charging current is less than a predetermined voltagethreshold.
 16. The electronic device of claim 9, further comprising: anoscillator that generates a plurality of clock pulses, wherein saidcharge pump receives said plurality of clock pulses and generates saiddriving signal which turns on said NMOSFET.
 17. The electronic device ofclaim 16, wherein said charging circuit powers off said oscillator andpowers on said pulse generator when a voltage of said battery is lessthan a threshold, and wherein said charging circuit powers off saidpulse generator and powers on said oscillator when said voltage isgreater than said threshold.